While taking a walk around Lake Temescal in the Berkeley, CA hills the other day, I noticed there were a few Cormorants in among the rest of the aquatic inhabitants of the lake.
Double-crested Cormorants (bottom center) among Ring-billed Gulls, Mallard ducks, Coots, Pigeons, and a stray female Wood Duck at Temescal Lake.
The Cormorants must have just finished their morning meal because most of them were intent on spreading their wings to dry off.
Any structure will do as a drying platform — and their big webbed toes provide stability on this tippy buoy. Note that Cormorants do not obey the “restricted area” warnings…
Cormorant feathers are “wettable”, unlike those of ducks and many other aquatic birds. This reduces their buoyancy as they dive for food. But the disadvantage of lacking the water-repellent oil on their feathers is that they have to dry off after a feeding bout in order to regain aerodynamic efficiency.
After I got too close to them on the beach, this pair swam off together, still holding their heads up at the same angle as when they stood on beach sand.
That strange head positioning, like a person tilting their head to look out through the bottom of their bifocal lens got me to wondering why they maintain their heads at that angle. Is there something peculiar about Cormorant vision related to their prowess as underwater fish catchers?
Some researchers speculated that Cormorants might possess superior underwater vision because they are so successful in capturing prey in 70-80% of their attempts. But, in fact, their visual acuity is no better than ours (without a dive mask) underwater, so how do they do it?
And how do they manage to catch anything in water like this?
Cormorant visual acuity is definitely poorer under conditions of low light, high turbidity, and in open water lacking any sort of underwater structures. In fact, they do far better in habitats where they can chase fish out of their hiding places. And they will strike at anything that moves — fish or other, moving objects — by lunging at the intended prey with a rapid neck extension, much like herons and egrets striking from above down into the water. So they are apparently cued by movement, not definitive shape of a prey item.
Cormorants can rotate their eyes independently forward, up, down, etc. But when they need to see what it at the end of their bill, the eyes turn inward, and with head slightly up, they can see straight ahead with binocular vision.
Once they have snatched something in one of their diving forays, they bring it up to the surface to inspect it, and powerful extra-ocular muscles move the eyes to focus on the tip of their bill to see what they have caught. If it’s edible, down the gullet it goes; if not, they spit it out.